1. Field of the Invention
The present invention relates to a color video camera, particularly to the method of generating luminance signals thereof.
2. Description of Related Art
FIG. 1 shows an arrangement of color filters of an image sensor described, for example, on pages 76 through 82 of xe2x80x9cNational Technical Reportxe2x80x9d, Vol. 31, No. 1, February 1985 composed by Matsushita Techno Research Co., Ltd. and published by Ohmsha Publishing Co., Ltd. In FIG. 1, Mg represents a pixel having a magenta color filter, G represents a pixel having a green color filter, Cy represents a pixel having a cyan color filter and Ye represents a pixel having a yellow color filter. FIG. 2 shows a part of a signal processing circuit of a color video camera which employs an image sensor consisting of these color filters arranged thereon. In FIG. 2, numeral 1 indicates a lens, numeral 2 indicates an image sensor, numeral 3 indicates a band-pass filter (BPF), numeral 4 indicates a detector, numeral 5 indicates a one horizontal period delay circuit (1HDLY), numeral 6 indicates a switching circuit and numeral 45 indicates a low-pass filter (LPF).
The operation will now be described below. In FIG. 2, incident ray on the lens 1 forms an image on the image sensor 2. In FIG. 1, an output signal from line n of the image sensor 2, consisting of a sequence of (Mg+Cy) and (G+Ye) being repeated, is denoted as Sn, and an output signal from line n+1 of the image sensor 2, consisting of a sequence of (Mg+Ye) and (G+Cy) being repeated, is denoted as S(n+1). Then Sn and S(n+1) are represented by the following equations.
Sn=Yn+Cnxc2x7sin(xcfx89t)+ . . .xe2x80x83xe2x80x83(1)
S(n+1)=Y(n+1)+C(n+1)xc2x7sin(xcfx89t)+ . . .xe2x80x83xe2x80x83(2)
Where xcfx89 is the carrier frequency of the color signal which corresponds to double the horizontal pixel width. Yn and Y(n+1) in equations (1) and (2) represent the luminance signal components of line n and line n+1, Cn and C(n+1) represent the color difference signal components of line n and line n+1, respectively, and are given by the following equations.
Yn=(Ye+G)+(Cy+Mg)=2R+3G+2Bxe2x80x83xe2x80x83(3)
Y(n+1)=(Ye+Mg)+(Cy+G)=2R+3G+2Bxe2x80x83xe2x80x83(4)
Cn=(Cy+Mg)xe2x88x92(Ye+G)=2Bxe2x88x92Gxe2x80x83xe2x80x83(5)
C(n+1)=(Ye+Mg)xe2x88x92(Cy+G)=2Rxe2x88x92Gxe2x80x83xe2x80x83(6)
The luminance signal components Yn, Yn+1 are obtained by passing the output of the image sensor 2 through the low-pass filter 45. The color difference signal components Cn, Cnxe2x88x921 are obtained by passing the output of the image sensor 2 through the band-pass filter 3 having a center frequency xcfx89 and a detector 4. The output of the detector 4 gives 2Rxe2x88x92G and 2Bxe2x88x92G appearing every two lines. These signals 2Rxe2x88x92G, 2Bxe2x88x92G which appear on every other line are synchronized by the one horizontal period delay circuit 5 and the switching circuit 6.
In the conventional color video camera as described above, an output signal of the image sensor is passed through a low-pass filter to remove the modulated components of the color signal and obtain a luminance signal, resulting in a problem of the harmonics of the luminance signals being attenuated. Although aperture correction has been made by enhancing the rising edge and falling edge of the signal to improve the resolution, it causes an impression of unnatural enhancement.
FIG. 3 shows a block circuit diagram illustrating the circuit of a color video camera employing a spatial offset of 3-chip CCD color camera which is described, for example, on pages 1079 through 1085 of the xe2x80x9cJournal of Television Engineering Associationxe2x80x9d, November 1986. In FIG. 3, numeral 51 indicates a lens, numeral 52 indicates a refracting prism which decomposes incident ray into three colors of red, green and blue, numeral 53, 54, 55 indicate image sensors, numeral 56 indicates a red signal amplifier, numeral 57 indicates a green signal amplifier, numeral 58 indicates a blue signal amplifier, numeral 68 indicates a low-pass filter, numeral 71 indicates an adder and numeral 73 indicates a demultiplexer. FIG. 4 shows the constitution of output signals with the conventional method of spatial offset of 3-chip CCD color camera. In FIG. 4, G represents the signal of a green pixel, and RB represents the composite signal of red and blue pixels. Letter p indicates the horizontal pixel width of the image sensor. Green image sensors and red, blue image sensors are arranged in the horizontal direction at intervals of a half pixel width.
The operation will now be described below. In FIG. 3, incident ray on the lens 51 is decomposed into red, green and blue by the refracting prism 52, with the light rays of respective colors forming images on image sensors 53, 54, 55. Each of the image sensors 53, 54, 55 mixes signals of the upper and lower adjacent pixels to give one signal output. The output signals R, G, B of the image sensors 53, 54, 55 are amplified by the red signal amplifier 56, the green signal amplifier 57 and the blue signal amplifier 58, respectively, so that the ratio of the output signals thereof becomes, in the case of NTSC system, R:G:B=0.30:0.59:0.11, to obtain Rxe2x80x2, Gxe2x80x2, Bxe2x80x2 signals. Rxe2x80x2 and Bxe2x80x2 are mixed in the adder 71 to obtain a signal RB which combines Rxe2x80x2 and Bxe2x80x2. The demultiplexer 73 switches alternately between Gxe2x80x2 and RB to produce an output of luminance signal, which is passed through the low-pass filter 68 to obtain a luminance signal Y. Consequently, the luminance signal Y is given by equation (7).
Y=0.30R+0.59G+0.11B=Rxe2x80x2+Gxe2x80x2+Bxe2x80x2=Gxe2x80x2+RBxe2x80x83xe2x80x83(7)
In the conventional method of spatial offset of 3-chip CCD color camera, the purpose is set at improving the resolution. Although there arises no problem in the case of such objects that have green signal and red-blue combined signal in similar proportions, but vertical lines appear in the case of objects which have significantly different proportions. Vertical lines have been reduced by passing the green signal and red-blue combined signal through a low-pass filter in the prior art, though it has a problem of causing attenuation of harmonics in the luminance signal. Thus resolution has been improved by enhancing the rising edge and falling edge of a signal for aperture correction, resulting in a problem of unnatural enhancement.
One object of the invention is to provide a color video camera which is capable of alleviating the attenuation of the harmonics of luminance signal.
Another object of the invention is to provide a color video camera made of low cost circuits which is capable of producing high quality image with less attenuation of the harmonics of luminance signal.
Further another object of the invention is to provide a color video camera which is capable of performing aperture correction without unnatural enhancement by means of simple circuits at a low cost.
In the color video camera of the invention, outputs from N kinds of spectral-response characteristics from one image sensor are passed through a low-pass filter to obtain outputs of N kinds from 1st through Nth, and the output signal of a pixel of the Kth spectral-response characteristic (1xe2x89xa6Kxe2x89xa6N) is multiplied by the ratio of the low-pass filter output, produced from the output of the image sensor at the coordinate of the pixel, to the Kth low-pass filter output at the coordinate of the pixel, thereby to obtain the luminance signal with lessened attenuation of the harmonics.
In another color video camera of the invention, N kinds signals obtained by adjusting the gain of the outputs from N kinds of image sensors are passed through a low-pass filter to obtain outputs of N kinds from 1st through Nth, and the output signal of the Kth pixel (1xe2x89xa6Kxe2x89xa6N) is multiplied by the ratio of the low-pass filter output, produced from the output of the image sensor at the coordinate of the pixel, to the Kth low-pass filter output at the coordinate of the pixel, thereby to obtain the luminance signal with lessened attenuation of the harmonics.
In further another color video camera of the invention, outputs from N kinds of spectral-response characteristics from one image sensor are passed through the first low-pass filter to obtain outputs of N kinds from 1st through Nth, and the output signal of a pixel of the Kth spectral-response characteristic (1xe2x89xa6Kxe2x89xa6N) is multiplied by the ratio of the first low-pass filter output, produced from the output of the image sensor at the coordinate of the pixel, to the Kth output of the first low-pass filter at the coordinate of the pixel, thereby to calculate the first signal, which is fed to the band-pass filter to obtain a second signal for aperture correction, the image sensor output is fed to the second low-pass filter to obtain a third signal and the second signal and the third signal are combined to obtain a luminance signal component, thereby performing aperture correction without unnatural enhancement.
In further another color video camera of the invention, N kinds signals obtained by adjusting the gain of the outputs from N kinds of image sensors are passed through the first low-pass filter to obtain outputs of N kinds from 1st through Nth, and the output signal of the Kth pixel (1xe2x89xa6Kxe2x89xa6N) is multiplied by the ratio of the first low-pass filter output, produced from the synthesized signals of the pixels of the respective kinds at the coordinate of the pixel, to the Kth output of the first low-pass filter at the coordinate of the pixel, to calculate the first signal which is fed to the band-pass filter to obtain the second signal for aperture correction, synthesized signal produced from N kinds of signals by adjusting the gain of the outputs of each image sensor is fed to the second low-pass filter to obtain the third signal, and the second signal and the third signal are synthesized to obtain the luminance signal, thereby performing aperture correction without unnatural enhancement.
The low-pass filter used in this invention may either be a one-dimensional low-pass filter consisting of a plurality of bit-shift circuits and an adder, or a two-dimensional low-pass filter consisting of a plurality of bit-shift circuits and a plurality of adders.
Use of a lookup table for division, a lookup table for logarithm, a lookup table for power or the like simplifies the constitution of the signal processing circuit.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.